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Difference Amplifier Forms Heart of Precision Current Source

Precision current sources provide a constant current in many applications, including industrial process control, instrumentation, medical equipment, and consumer products. For example, current sources are used to provide excitation for resistance-temperature detectors (RTDs) in process-control systems; to measure unknown resistors, capacitors, and diodes in digital multimeters; and to drive 4 mA to 20 mA current loops, which are widely used to transmit information over long distances.

Precision current sources have traditionally been built using op amps, resistors, and other discrete components—with limitations due to size, accuracy, and temperature drift. Now, high-precision, low-power, low-cost integrated difference amplifiers, such as the AD8276, can be used to achieve smaller, higher performance current sources, as shown in Figure 1. The feedback buffer uses amplifiers with low offset and low bias current, such as the AD8538, AD8603, AD8605, AD8628, AD8655, AD8661, AD8663, OP177, or OP1177, depending on the required current range.

Figure 1. Difference amp and op amp form precision current source.

The output current can be calculated as follows:

If Rg1 = Rg2 = Rf1 = Rf2, the equation can be reduced to:

The maximum output current is limited by the op amp input range, diff amp output range, and diff amp SENSE pin voltage range. The following three conditions must be met:

within op amp input range

within SENSE pin voltage range = 2×(–Vs) – 0.2 V to 2×(+Vs) – 3 V

within AD8276 output voltage range = –Vs + 0.2 V to +Vs – 0.2 V

The SENSE pin can tolerate voltages almost twice as large as the supplies, so the second limitation will be very loose. The wide 2.5-V to 36-V supply range makes AD8276 ideal for many applications. The maximum gain error of A- and B-grades is 0.05% and 0.02%, respectively, allowing current sources with up to 0.02% accuracy to be achieved.

Configuration Variations

For cost-sensitive applications that can tolerate a little more error, the circuit can be simplified by removing the feedback buffer, as shown in Figure 2.

With

the output current is:

For

Figure 2. Simplified circuit eliminates feedback amplifier.

If the required output current is less than 15 mA—the output capability of AD8276—then the boost transistor can be eliminated, as shown in Figure 3. If both low current and reduced accuracy are acceptable, the simpler, lower-cost configuration of Figure 4 can be employed.

External resistors R1 and R2 should have ultra-high-precision and matching, or the output current will vary with the load, an error that cannot be corrected with software.

Peripheral Components

The input voltage, VREF, can be a DAC output, voltage reference, or transducer output. If a programmable current source is needed, precision 14- or 16-bit DACs, such as the AD5640, AD5660, AD5643R, and AD5663R are recommended. For voltage references, the precision ADR42x, ADR44x are recommended for higher performance; the ADR36x is recommended for low power; the AD158x and ADR504x are recommended for low cost; and the ADR82x integrated op amp and voltage reference is recommended for small size.

The reference can connect to either the inverting or the non-inverting input of the AD8276. If using the non-inverting input, the common-mode voltage will be

and the output current will be

If using the inverting input, the common-mode voltage will be

and the output current will be

When using the inverting input, a buffer amplifier is required; the non-inverting input is thus recommended for simplicity.

Transistor Selection

When selecting the boost transistor, make sure that VC is higher than the power supply voltage and IC is higher than the desired output current. Low cost devices, such as 2N3904, 2N4401, and 2N3391 are recommended. For lower current, the transistor is not needed.

Experimental Bench Results and Analysis

The input voltage versus output current measured using the circuit of Figure 1 is shown in Figure 6. The AD8276 and AD8603 are powered by +5 V. The tolerance of R1 is 0.1%. The transistor is a 2N3904. The reference was swept from 0.05 V to 1.20 V with 0.01-V steps. The input range is limited by the power supply and the AD8603 input range.

The maximum error is 0.87% and the average is 0.10%. The current sense error is limited by the external resistors. Higher accuracy resistors will produce higher accuracy current sources.

Figure 6. Test result using diff amp and feedback amp

Conclusion

The AD8276 difference amplifier—with its low offset voltage, low offset voltage drift, low gain error, and low gain drift, and integrated resistors—can be used to implement accurate, stable current sources. Its wide power supply range (2.5 V to 36 V) allows it to accommodate a wide range of loads. Its space-saving 8-lead MSOP package and its low power dissipation make it ideal for battery powered and portable systems. Implementing a precision current source with a difference amplifier can reduce PCB area, simplify layout, decrease system cost, and improve reliability.

Authors

Neil Zhao

Neil Zhao has worked as a field applications engineer with the China Applications Support Team at ADI for one year. He provides technical support for horizontal analog products across China. In January 2008, Neil graduated from Beihang University with a master’s degree in communications and information systems. He has published several articles.

Reem Malik

Reem Malik is an applications engineer in the Integrated Amplifier Products (IAP) Group in Wilmington, MA. She supports customers in the instrumentation, industrial, and medical areas and is responsible for thermocouple amplifiers and precision difference and differential amplifiers. Reem holds BSEE and MSEE degrees from Worcester Polytechnic Institute. She joined Analog Devices in June 2008.

Wenshuai Liao

Wenshuai Liao is a marketing engineer in ADI’s Linear Products Group (LPG) located in Wilmington, Mass. After earning a master’s degree in optical engineering from Tsinghua University, Wenshuai spent three years as a 3G Node B RF engineer at Datang Telecommunications Group. He joined ADI in August 2002.

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